AssetManager无法从资产文件夹中检索文件

Question

我正在尝试从测验应用程序的片段中的资产文件夹中加载.png文件，但它始终返回空。

在调用.getAssets（）之前，我已经检查了该片段是否已附加到Activity上，并且在调用中包含了.getActivity（）。错误消息本身是由于尝试从Assetmanager打开文件而引起的。

这是给我错误的代码段

    private void loadNextSign() {
            // get file name of the next sign and remove it from the list
            String nextImage = quizSignList.remove(0);
            correctAnswer = nextImage; // update the correct answer
            answerTextView.setText(""); // clear answerTextView

            // display current question number
            questionNumberTextView.setText(getString(
                    R.string.question, (correctAnswers + 1), SIGNS_IN_QUIZ));

            // extract the region from the next image's name
            String region = nextImage.substring(0, nextImage.indexOf('-'));

            // use AssetManager to load next image from assets folder
            System.out.println(isAdded());
            AssetManager assets = getActivity().getAssets();

            // get an InputStream to the asset representing the next sign
            // and try to use the InputStream
           try (InputStream stream =
                         assets.open( region + "/" + nextImage + ".png")) {
                // load the asset as a Drawable and display on the signImageView
                Drawable sign = Drawable.createFromStream(stream, nextImage);
                signImageView.setImageDrawable(sign);

                animate(false); // animate the sign onto the screen
            }
            catch (IOException exception) {
                Log.e(TAG, "Error loading " + nextImage, exception);
            }

Logcat

05-14 16:48:01.386 18314-18314/com.example.signquiz E/SignQuiz Activity: Error loading RoadSigns-Slippery_Roads
    java.io.FileNotFoundException: RoadSigns/RoadSigns-Slippery_Roads.png
        at android.content.res.AssetManager.openAsset(Native Method)
        at android.content.res.AssetManager.open(AssetManager.java:313)
        at android.content.res.AssetManager.open(AssetManager.java:287)
        at com.example.signquiz.MainFragment.loadNextSign(MainFragment.java:83)
        at com.example.signquiz.MainFragment.resetQuiz(MainFragment.java:273)
        at com.example.signquiz.MainActivity.onStart(MainActivity.java:70)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1237)
        at android.app.Activity.performStart(Activity.java:6253)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

我是Java和android编程的新手，但据我了解，这应该可以正常工作。我在做什么错了？